Esther Perel is explaining why, as a woman, putting sex on your “to-do” list isn’t just OK, but might save your marriage. “Honestly, it’s a wonderful idea,” the Belgian psychotherapist and bestselling author tells me, grave and glamorous in the study of her Manhattan apartment. “People are always horrified when you tell them that you have to schedule sex, because they want it to be spontaneous and to fall from the heavens while you’re folding the laundry. But sex only happens if you make it happen.”

Because female sexual desire is “responsive” rather than “initiative”, the 59-year-old goes on, “putting it on a ‘to-do’ list and then putting yourself in the mood is saying ‘This is important to me’, in just the same way as a woman might with going to the gym. Nobody says, ‘You should just want to go and run’, do they? So it’s not about seeing it as a chore, but making time for yourself and the experience. It’s about assigning value to it.”

The value of intimacy — from sex itself to the most elusive vagaries of the human heart — has been at the core of Perel’s work for more than 20 years. Indeed, the daughter of two Polish-born Holocaust survivors believes her understanding of its importance has helped her own 35-year marriage, to psychologist husband Jack Saul, endure.

Her bestselling 2006 book, Mating in Captivity, which examined our conflicting need for both security and erotic novelty, was translated into 26 languages, and her TED talk, “The Secret to Desire in a Long-Term Relationship”, has been viewed more than 15 million times. But it’s the knowledge and research Perel has amassed on infidelity over the years, through her work as a couples’ therapist, that’s most compelling.

Despite (or perhaps because of) it being banal, easy and something Perel estimates 80 per cent of us have been directly affected by, our morbid fascination with why, how and with whom we cheat never wanes.Seven million Britons tuned into the BBC’s Doctor Foster last week to see whether a suburban mother would take her cheating husband back. And according to one US critic, the 20 million copies sold of Gillian Flynn’s Gone Girl were largely down to us all slowing down to get a good look at “the lurid crime scene” that was Amy and Nick Dunne’s unfaithful marriage.

Rosamund Pike as Amy Dunne in the film adaptation of Gone Girl.GIPHY

Readers of Perel’s new book, The State of Affairs, will experience that same perfect split of horror and what I can only describe as erotic vertigo — because Perel is defiantly non-judgmental — showing us the appeal and the “benefits” of extra-marital affairs as well as the chaos they can cause, and unveiling some surprising statistics as she does so, the most significant being the 40 per cent jump in female infidelity since 1990.

“Male figures have held steady,” Perel tells me with a wry smile. “But despite the freedom women now have — to choose who they marry, to have a relationship that is based on connection and pleasure, not just duty, to have an economic independence, no longer be beholden to having 10 children, and crucially to be able to divorce if they are unhappy — they are ‘transgressing’ more than ever.” When Perel asks the women on her couch why this is, the answer is always the same: “Because I feel alive.”

“So if you really want to understand women’s sexuality, don’t look at what they do in their marriages, look at how they conduct their affairs. In their marriages, women are doing what they’ve been told to do, following the institutional mandate and struggling to hold on to their sense of selves. In their affairs, they are following their own desires and often feeling a greater sense of authenticity in themselves.”

That sense of vibrancy is in direct contrast to the “numbness to their own pleasure and longings” women can feel beneath the weight of their responsibilities. “The caretaking and devotion that a woman has to her children, family, partner and home often means that, for years, she thinks: ‘I have no time to think about myself or even go to the hairdresser.’”

In their affairs, they are following their own desires and often feeling a greater sense of authenticity in themselves

That children have been “so sentimentalised” in modern relationships is only helping to kill the intimacy our parents and grandparents had, says Perel. “And I would go further and say: ‘Have sex for your kids.’ Because you’re not being selfish when you have sex with your husband or wife; you’re actually saving your family by maintaining that erotic intimacy. All that erotic energy is being redirected into people’s children now, so the children get to be playful and wear new clothes and try out new activities, but the adults wear the same discoloured sweat pants and do the same old thing year after year.”

When women experience what Perel describes as their “now me” moment — a moment that doesn’t exist in the male mind, she points out — and embark on an affair, “interestingly, they are not too tired or stressed, but have all the time in the world: they are powered by their own transgression. As one woman once said: ‘It’s cheaper than a facelift and much more effective than an anti-depressant.’ ”

I tell Perel that I’ve always believed happy men can still cheat, whereas when a woman is unfaithful there’s usually something missing in her life, and she agrees with the first part (“a man can absolutely be happy and unfaithful”), but dismisses the second as a culturally accepted manner of thinking. “There are reasons why women have always needed more serious motivations in order to brave the risks of what they stand to lose. Because the consequences for women are still not the same as they are for men. The same double standard still exists today: the word ‘home-wrecker’ doesn’t exist in the masculine. And there is no ‘other man’ — only the ‘other woman’.”

This may be one of the only aspects of marriage that is unchanged over the centuries. In every other way, “the couple has undergone an extreme makeover”, maintains Perel. “We used to marry and have sex for the first time; now we marry and stop having sex with others. We used to think monogamy meant one person for life; now we see monogamy as one person at a time. It used to be ’till death do us part’; today, you marry until love dies.”

Because we live in the age of happiness and entitlement? “Absolutely. And because whereas we used to choose between three people in the village, we now have this digital online forest with bountiful flora. And only when you find The One can you stop looking.”

It used to be 'till death do us part'; today, you marry until love dies

Perel believes it’s this emphasis on “The One” that’s setting so many marriages up to fail. “Relationships are crumbling under the weight of people’s expectations. But people won’t give up that idea. He or she has to be your passionate lover, intellectual equal, best parent, best friend and be able to maintain a sense of mystery and awe and transcendence besides,” she explains, deliberately employing religious language. “The soulmate is what people used to seek out in religion, but romantic love is the new religion. Every big ideology of the 19th century has come and gone: communism, socialism, Marxism. The only one that’s still sturdy as hell is romanticism. And when you think that you’re The One and infidelity tells you that you’re not, it can be one of the most painful things.”

Betrayal doesn’t have to end a relationship, however. And as the child of two people who were “betrayed not by their partners, but by humanity”, Perel is adamant about this. “How people can come out of a crisis and remain connected, and how you can maintain a sense of aliveness after total destruction, are the key themes of my work and a part of my fabric. I’ve always seen myself as a symbol, not of survival, but revival.”